Reaction electrophilic

Trivalent carbenium ions are the key intermediates in electrophilic reactions of Tt-donor unsaturated hydrocarbons. At the same time, pen-tacoordinated carbonium ions are the key to electrophilic reactions of cr-donor saturated hydrocarbons through the ability of C-H or C-C single bonds to participate in carbonium ion formation. 

This realization led me to study related possible intermolecular electrophilic reactions of saturated hydrocarbons, Not only protolytic reactions but also a broad scope of reactions with varied electrophiles (alkylation, formylation, nitration, halogenation, oxygenation, etc.) were found to be feasible when using snperacidic, low-nucleophilicity reaction conditions. 

Apart from Bronsted acid activation, the acetyl cation (and other acyl ions) can also be activated by Lewis acids. Although the 1 1 CH3COX-AIX3 Friedel-Crafts complex is inactive for the isomerization of alkanes, a system with two (or more) equivalents of AIX3 was fonnd by Volpin to be extremely reactive, also bringing abont other electrophilic reactions. 

Electrophilic reaction can account for the additions of morpholine or phosphonyl (Scheme 14), or... 

As in most electrophilic reactions, the abiUty to stabilize the positive charge generated by the initial addition strongly affects the relative rates. MX reacts faster than OX and PX because both methyl groups work in conjunction to stabilize the charge on the next-but-one carbon. Sulfonation was, at one time, used to separate MX from the other Cg aromatic isomers. MX reacts most rapidly to form the sulfonic acid which remains in the aqueous phase. The sulfonation reaction is reversible, and MX can be regenerated. 

There are three general reactions of perfluoroepoxid.es pyrolyses (thermal reactions), electrophilic reactions, and by far the most important, reactions with nucleophiles and bases. 

Electrophilic Reactions. Perfluoroepoxides are quite resistant to electrophilic attack. However, they react readily with Lewis acids, for example SbF, to give ring-opened carbonyl compounds (20—22) (eq. 2). 

The stmcture of the ketones produced from unsymmetrical internal perfluoroepoxides has been reported (5). The epoxide ring may also be opened by strong protic acids such as fluorosulfonic acid or hydrogen fluoride at elevated temperatures (23—25). The ring opening of HFPO by sulfur trioxide at 150°C has been interpreted as an example of an electrophilic reaction (26) (eq. 3). 

Reactions of the Aromatic Ring. The aromatic ring of hydroxybenzaldehydes participates in several typical aromatic electrophilic reactions. Ha.logena.tlon, Chlorination and bromination yield mono- and dihalo derivatives, depending on reaction conditions. Bromination of / -hydroxy-benzaldehyde in chloroform yields 65—75% of the product shown (39). 

The earliest reported reference describing the synthesis of phenylene sulfide stmctures is that of Friedel and Crafts in 1888 (6). The electrophilic reactions studied were based on reactions of benzene and various sulfur sources. These electrophilic substitution reactions were characterized by low yields (50—80%) of rather poorly characterized products by the standards of 1990s. Products contained many by-products, such as thianthrene. Results of self-condensation of thiophenol, catalyzed by aluminum chloride and sulfuric acid (7), were analogous to those of Friedel and Crafts. 

As conjugated systems with alternating TT-charges, the polymethine dyes are comparatively highly reactive compounds (3). Substitution rather than addition occurs to the equalized TT-bond. If the nucleophilic and electrophilic reactions are charge-controHed, reactants can attack regiospeciftcaHy. 

Other Electrophilic Reactants. ReversibHity of the electrophilic reactions enables substituted dye derivatives to be obtained. Thus, the halogenation of cyanines, oxonoles, and merocyanines has been studied (3,65,66). Halogen atoms are mobHe in the polymethine chain, and the derivatives themselves can function as halogenation reagents. 

Nucleophilic Reagents. In contrast to electrophilic reactions, nucleophiles attack positively charged, even carbons ia the chain. The reactions lead to the exchanging of substituents or terminal residues. Thus, SR and OR groups, or halogen atoms can be exchanged by other suitable nucleophiles (4,69,70), for example, by aniline ... 

Mesitylene continues to be of considerable iaterest as a research chemical. Its ttilateral stmcture, extremely high reactivity for electrophilic reactions, and the fact that large suppHes are available poiat to good growth potential. 

The propylene double bond consists of a (7-bond formed by two ovedapping orbitals, and a 7t-bond formed above and below the plane by the side overlap of two p orbitals. The 7t-bond is responsible for many of the reactions that ate characteristic of alkenes. It serves as a source of electrons for electrophilic reactions such as addition reactions. Simple examples are the addition of hydrogen or a halogen, eg, chlorine ... 

Many other reactions of ethylene oxide are only of laboratory significance. These iaclude nucleophilic additions of amides, alkaU metal organic compounds, and pyridinyl alcohols (93), and electrophilic reactions with orthoformates, acetals, titanium tetrachloride, sulfenyl chlorides, halo-silanes, and dinitrogen tetroxide (94). 

Nitration of 4-(2-thienyl)- (301) and 4-(3-thienyl)-pyrazoles (302) mainly occurs on the thiophene ring, but when acetyl nitrate is used as the nitration agent small quantities of products nitrated on the pyrazole ring are isolated (position of the nitro group uncertain) (80CS( 15)102). Pyrazol-l -ylpyridines (303) undergo electrophilic reactions (bromination, chlorination and nitration) preferentially in the pyrazole ring. Thus, the nitration of (303 R = R = = H) either with a mixture of nitric acid and sulfuric acid at 10-15 °C or with... 

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